This invention relates in general to reduction and defibering machinery and methods. More specifically, it relates to an improved pulper rotor-stator that defibers a wide range of difficult materials, that can increase selectively the agitation flow or defibering and recirculation flow of a stock, and can do so with an extended service life.
Pulpers are typically used in the paper and pulp industry to reduce a stock material, such as wood pulp, into a watery slurry suitable for making paper. The stock material is added to a tank of water incorporating a rotor-stator pair where the stock material is broken down into fibers of a suitable size and consistency to make the desired paper product.
Until the introduction of the pulper described and claimed in U.S. Pat. No. 4,365,761, it was impossible to defiber a variety of materials in pulpers. These "unconventional" or "difficult" materials include cotton, hemp, flax, rag, leather, high wet strength papers, synthetic fibers, sheets of stock formed of fibrous material bound by adhesives, and in particular, the high wet strength board known as "shoe board". The defibering difficulty affects not only the cost and quality of the final paper product, but also the ability to recycle old paper products into new paper products, thereby avoiding landfill and saving trees.
The pulper described in U.S. Pat. No. 4,365,761 uses a rotor and stator operating at close clearance (e.g. 0.010 inch). The rotor and stator are configured to "acquire" and cut difficult materials with a scissor-like action at a size reduction interface having a truncated conical geometry. This interface is defined in part by a series of generally triangular segments, or lobes, of the stator, which each curve along the outer edge of a generally circular base. Each lobe also inclines inwardly. The inner surface of these lobes defines a conical, as opposed to a cylindrical, interface. An outer cutting edge of blades, on the base of the rotor define the inner boundary of this interface. Scissoring occurs between these blade cutting edges and the leading edge of each triangular stator lobe. Once acquired and reduced to a sufficiently small size, the material is defibered in the attrition zone of the pulper between the lobes and the outer edges of the blades.
This interface reduces in size large stock such as large sheets of shoe board and woven or natural cotton into "flakes" that can be further defibered without clogging. To date the '761 pulper, sold by Bolton-Emerson Americas, Inc. under the registered trade designation "Tornado.RTM." is the only commercial pulper which can handle such materials. The Tornado.RTM. pulper is believed to be used to prepare the slurries that make about half of the paper currencies now in circulation throughout the world.
The energy input to the Tornado.RTM. pulper is used to reduce in size and defiber the material, to recirculate the flow of defibered stock back to the pulper (or, alternatively, to transfer stock downstream on a continuous basis), and to agitate the stock held in the tank using a toroidal flow. The pulper must also effectively deal with problems such as the tendency of some stock to float or settle in the tank ("submergence"), plugging of the defibering mechanisms, and "slugging" due to the rapid introduction of a mass of difficult material to the acquisition and attrition zones of the rotor-stator pair.
While the '761 Tornado.RTM. rotor-stator design works well, it is adapted for use in conjunction with a standard size tank (7.5 foot inside diameter) and designed principally for defibering materials used in making paper and cardboard products. If it is placed on a larger size tank, (e.g., one with a 12 foot inside diameter), more power is required to produce sufficient agitation in the tank to mix the stock and draw it to the pulper. But more importantly, there is now a demand for a pulper which can effectively defiber a much wider range of materials to produce products very different from paper.
For example, it is now desired to pulverize municipal solid waste (MSW) so that it can be fed to a reactor to make high energy liquid fuel to fire power plants. It is also desired to reduce MSW to fuel pellets for power generation. There is also an interest in "atomizing" scrap leather for conversion into pure protein for animal and human consumption. Another application is defibering old polypropylene carpeting and carpet scraps so that the fibers can be re-pelletized to make new plastic products. Agricultural products such as hemp, flax, kenaf, and straw, if defibered, can be made into various paper products. It is also desired to transform (i) "trash" fish and fish parts into liquid fertilizer, (ii) manure into fuels or fertilizer, (iii) old books and magazines into mulch, and (iv) even process the contents of old landfills in order to reclaim the land.
These different materials and different end uses for a defibered product each present special processing problems. To produce an efficient and rapid pulping--without heat and chemicals--the pulper design must somehow accommodate significant differences in the raw materials, their flow characteristics, and their submergence in the tank as well as operating parameters such as consistency of the defibered stock, production rate, the amount of agitation required, and the recirculation flow rate. For example, where larger tanks are already installed, or they are needed to handle the raw material, more of the available power must be directed to agitation. Stronger raw materials, on the other hand, require more power for ripping and shredding. Homogeneity of the treatment to provide a high degree of uniformity of the size of the processed fibers is important in other end use applications. Different materials and end applications for the stock also influence the degree to which the rotor-stator pair must meet the design needs of 1) a scissor-like shearing action 2) a refiner-like defibering action, and 3) extraction (of small fibers to an extraction chamber and then to a recirculation line or to downstream equipment). The rotor-stator design must also be able to alter the distribution of power among recirculation, agitation of the stock within the tank, and defibering. To date, known pulper construction cannot meet these various design considerations for the many possible applications, particularly those outside of the paper and pulp industry.
In addition, known pulpers continue to suffer from wear. Wear is particularly troublesome in dealing with highly abrasive materials such as those used in certain flooring bases. Replacement of rotor-stator components requires costly production down-time as well as the cost of replacement parts.
Further, it may be desired to control the operation of a known Tornado.RTM. pulper to defiber materials "coarsely", to an unusually long fiber length. The rotor-stator interface is essentially a highly effective moving screen that rapidly reduces material to a defibered state. Under normal operating conditions, coarse fibers will be reduced in size. There is also a need to defiber to unusually short fiber lengths--a "fine" defibering--and within reasonable commercial production parameters, e.g., without repeated recycling or pre-pulper or post-pulper processing.
It is, therefore, a principal object of the present invention to provide a rotor-stator assembly for, and method of operation of, a pulper which can defiber a wide range of difficult raw materials for the production of a wide range of end products.
Another principal object of the present invention is to increase pulper production without sacrificing defibering effectiveness or energy usage efficiency.
Yet another object is to provide either increased agitation or a greater effectiveness in defibering and increased recirculation.
A further object is to provide a rotor design and method of operation of a rotor-stator pair which can readily compensate for wear at the rotor-stator interface while maintaining the aforementioned operational advantages, even when processing highly abrasive materials.
Still another object is to provide a rotor-stator assembly and method of operation which can provide greater homogeneity of treatment and thus produce a stock with a high degree of consistency in the length of the fibers.
A still further object of the invention is to provide a mechanically simple and reliable way to defiber to fine and coarse fiber lengths without clogging and without costly additional processing or repeated recycling, and without unusual or time sensitive modes of operation of the pulper.